1. Field of the Invention
The invention relates to a communication system comprising base stations and mobile stations for communication among mobile stations and/or among mobile stations and wire-bound subscribers, in which system a plurality of radio channels in the direction from mobile station to base station (uplink radio channels) and a plurality of radio channels in the direction from base station to mobile station (downlink radio channels) are assigned to each other in pairs as duplex radio channels, and in which for seizing a duplex radio channel or for a shift to a trunked mobile radio system signalling takes place between base station and mobile station over at least one control channel.
2. Description of the Related Art
Mobile radio systems for covering a rather large coverage area have been designed as cellular radio networks for a long time. In each cell of such a radio network there is at least one stationary base transceiver station which maintains the radio connection in the full-duplex mode with the mobile stations located in its radio cell. By means of signalling methods on a control channel it is ensured that never more than one mobile user at a time occupies a full-duplex radio channel.
For closed user groups there are also what is commonly referred to as trunked mobile radio systems in which, in the half-duplex mode, the individual users all access one radio channel. Access to this radio channel is ensured by suitable signalling methods by which in the case of group calls, only one user at a time of a group of users is in a position to speak, but users in the same user group can hear him/her. Such trunked mobile radio systems are highly frequency-efficient, because a group of users need to have only a single radio channel due to the half-duplex mode.
Mobile radio systems and trunked mobile radio systems are radio systems operating in mutually different frequency bands and also having different geographical areas. For example, trunked mobile radio systems used in Germany nowadays are even regionally restricted to specific large cities and their neighbouring areas.
WO 92/08326 has disclosed a method by which it is possible with only one radio receiver to take part in both a mobile radio system and a trunked mobile radio system, while a shift between the two systems can be made as desired. For this purpose, different access numbers are used on the control channels monitored by the mobile stations, from which numbers a mobile station can distinguish in which of the two systems a call for that mobile station is being received, and if in the trunked mobile radio system whether a group call to its own user group is being made.